HR: 1340h
AN: T33B-0530    [Abstracts]
TI: The Taiwan-Ryukyu subduction-collision complex, II: Tomographic imaging of seismic velocity structure beneath the southernmost Ryukyu Arc and its implications for slab buckling, dehydration, and melting
AU: * Chou, H
EM: d92224001@ntu.edu.tw
AF: Institute of Geosciences, National Taiwan University, No.1, Sec. 4, Roosevelt Road, Taipei, Tw 106 Taiwan
AU: Hung, S
EM: shung@ntu.edu.tw
AF: Institute of Geosciences, National Taiwan University, No.1, Sec. 4, Roosevelt Road, Taipei, Tw 106 Taiwan
AU: Kuo, B
EM: byk@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, 128, Sec. 2, Academia Road Nankang, Taipei, Tw 115 Taiwan
AU: Zhao, D
EM: zhao@sci.ehime-u.ac.jp
AF: Geodynamic Research Center, Ehime University, 2-5 Bunkyo-cho, Matsuyama, 790-8577 Japan
AU: Chiao, L
EM: chiao@ntu.edu.tw
AF: Institute of Oceanography, National Taiwan University, No.1, Sec. 4, Roosevelt Road, Taipei, Tw 106 Taiwan
AU: Wu, Y
EM: drymwu@ntu.edu.tw
AF: Institute of Geosciences, National Taiwan University, No.1, Sec. 4, Roosevelt Road, Taipei, Tw 106 Taiwan
AB: The southwestern termination of the Ryukyu trench against the Eurasian Plate lies at the junction with the Taiwan mountain belt as a result of the arc-continent collision. The Taiwan-Ryukyu collision-subduction contact accompanied by arc-backarc magmatism renders offshore northeast Taiwan a rather complex and unique place for the study of plate boundary processes. A sequence of studies began with first integrating large seismic datasets from the Taiwan and Japan networks to determine totally 5667 earthquake hypocenters with sufficient accuracy. The relocated seismicity illuminated the characteristic buckling of the Philippine Sea Plate (PSP) at the depths of 50-100 km near its western end. The double-planed seismic zone (DSZ) occurred 15-20 km apart distributed over the depth range of 40-80 km. In this sequel study, we utilize the arrival time data from the precisely-located events to obtain the P and S wave velocity (V_p and V_S) structures underneath the junction area. The northward descending PSP outlined by ~2--8% higher V_p and V_S anomalies is well resolved from the surface down to 200 km depth. The sudden shallowing of the dip angle above 90 km depth as revealed near the western edge of the high-velocity slab image confirms its mildly folded geometry deformed by horizontal compression from oblique plate convergence. The upper layer of the DSZ mostly coincides with the low velocity region which probably corresponds to the subducted former oceanic crust. The low V_p, low V_S and high Poisson's ratio structure above the 60-100 deep portion of the downgoing slab appears tied to the water flux into the mantle wedge from dehydration of subducted marine sediments and altered oceanic basalts. The lack of the high Poisson's ratio anomalies in the sub-arc mantle gives no clear evidence for the presence of partial melting beneath the Yonaguni Island at the southern tip of the Ryukyu island arc. The associated low V_p and V_S bodies confined in the shallow depths manifest its sedimentary origin.
DE: 8138 Lithospheric flexure
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8178 Tectonics and magmatism
DE: 8180 Tomography (6982, 7270)
DE: 8185 Volcanic arcs
SC: Tectonophysics [T]
MN: Fall Meeting 2005